The present invention relates to optical pickups for reading out data from high density recorded media, for example digital audio disks, and more particularly relates to structures for tracking data on the disk and for focusing optical systems of the optical pickups.
Apparatus for reading out data optically from a disk is already known and an example is shown in FIG. 1. The apparatus has an optical system 100, a servo driver 102 for moving optical system 100 to track data on the disk and focus optical system 100 with respect to the data, and a servo amplifier 104. A numeral 106 denotes a recorded medium, for example a disk.
Optical system 100 has a light source 108 which may be a laser emitting diode, a collimator lens 110, a polarizing beam splitter 112, a quarter-wave plate 114, an object lens 116, a focusing lens 118 and a photo detector 120. These parts of optical system 100 are assembled as a unit. Light from source 108 passes through collimator lens 110, polarizing beam splitter 112 and quarter-wave plate 114, and is then focused by objective lens 116 to make a very small light spot on a surface of recorded medium 106. Reflected light containing information from disk 106 travels through object lens 116, quarter-wave plate 114 and polarizing beam splitter 112 in a reverse direction so that it passes through focusing lens 118 and is focused on photo detector 120 as a light spot. Photo detector 120 converts the light into an electric signal on line 122 which signal includes a data signal and a location signal. The location signal includes tracking error information and focusing error information. The location signal is applied to amplifier 104 on line 124 wherein it is amplified. The amplified location signal is then transmitted to servo driver 102 for adjusting optical system 100 to minimize the tracking error and the focusing error.
In the known optical pickup mentioned above, optical system 100 including objective lens 116, focusing lens 118, photo detector 120, light source 108, etc. is formed as a single unit. Optical system 100 is connected to a frame of the optical pickup by resilient supporting members (not shown). Further, servo driver 102 moves the entirety of optical system 100 electromagnetically. Therefore, the supporting members must be relatively strongly resilient, because of the total weight of optical system 100. As a result, a relatively large amount of electric power is required for tracking and focusing. Therefore, the optical pickup has low driving sensitivity and degraded focusing and tracking control characteristics. Further, the large power consumption for servo driving generates heat which deteriorates the laser diode of light source 108, shortening its life.